In general, a plasma processing apparatus for performing a predetermined plasma processing on a substrate, such as a semiconductor wafer, a flat display panel or the like, includes an accommodation chamber (hereinafter, referred to as a “chamber”) for accommodating therein the substrate. In the plasma processing apparatus, a processing gas is introduced into the chamber through a gas introduction shower head functioning as a gas supply member and a high frequency power is applied thereto to produce a plasma from the processing gas, wherein the plasma is used in performing a plasma processing on the substrate.
A portion facing the chamber (to be referred as a “chamber-facing portion” hereinafter) of the gas introduction shower head is normally made of a flat plate having a plurality of gas holes for injecting the processing gas. However, when the high frequency power is applied to the chamber, an electric field tends to be concentrated at an outer periphery portion of a gas hole of the gas introduction shower head, which may cause an abnormal discharge. Such an abnormal discharge incurs damage on the substrate or constituent components disposed in the chamber. Specifically, crack, notch or the like may be developed on a surface of a semiconductor wafer employed as the substrate, or the constituent component may be damaged.
Thus, there has been known in the conventional plasma processing apparatus that a curved surface is formed at the outer periphery portion of the gas injection hole to prevent the concentration of the electric field and the abnormal discharge (e.g., see Japanese Patent Laid-open Application No. S59-4011).
Since, however, there are planar portions between gas holes in the chamber-facing portion of the gas introduction shower head of the conventional plasma processing apparatus, a flow of the processing gas injected through the gas holes is abated in the middle portions between the gas holes where the planar portions exist, so that the processing gas remains stagnant thereat. Particles produced in the chamber move towards where a gas viscous force caused by collisions with gas molecules of the processing gas injected through the gas holes, an ion viscous force caused by collisions with ions and an electrostatic force applied thereto are balanced with each other (FIGS. 8 and 9). Therefore, the particles are left stagnant in the middle portions between the gas holes, where the flow of the processing gas is abated and thus the gas viscous force becomes weak. Further, since radicals serving as precursors also remain stagnant in the middle portions between the gas holes as in the case of the particles, deposits are likely to be made on the middle portions and then peeled off therefrom to become foreign substances adhering on the semiconductor wafer. Moreover, accumulation of deposits may lead to changes in the reaction process in the chamber (memory effect).